1. Field of the Invention
This invention relates generally to a compressor that increases the pressure of a gas delivered from a source to a target and, more particularly, to a thermal hydrogen compressor that increases the pressure of hydrogen gas delivered from a hydrogen source to a high pressure tank, where the compressor includes a series of pressure vessels that are selectively heated and cooled in a manner that causes the gas to flow from the source to the tank through the pressure vessels with an increasing pressure from pressure vessel to pressure vessel in the series.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Typically hydrogen is stored in a compressed gas tank under high pressure on the vehicle to provide the hydrogen necessary for the fuel cell system. The pressure in the compressed tank can be upwards of 700 bar (70 MPa). In one known design, the compressed tank includes an inner plastic liner that provides a gas tight seal for the hydrogen, and an outer carbon fiber composite layer that provides the structural integrity of the tank. Because hydrogen is a very light and diffusive gas, the inner liner and the tank connector components, such as O-rings, must be carefully engineered in order to prevent leaks. The hydrogen is removed from the tank through a pipe. At least one pressure regulator is typically provided that reduces the pressure of the hydrogen within the tank to a pressure suitable for the fuel cell system.
A network of refueling stations will need to be provided as fuel cell vehicles become more popular and commercially available. Such a network of refueling stations will initially be provided in a limited manner, where urban centers will probably be the first to get such refueling stations and the number of fueling stations will expand from there. Because of this limited number of refueling stations, it has been proposed that a home fueling appliance be provided that generates hydrogen gas, and provides the hydrogen gas to the vehicle storage tanks at high pressure. The home fueling appliance can be used to top off the fuel storage system so that the consumer starts every morning with a full tank of hydrogen. Such home fueling appliance will need to be relatively inexpensive and be of a reasonable size.
Commercially available electrolyzers can be used to break water into its hydrogen and oxygen components, where the oxygen will typically be discarded. State of the art electrolyzers are typically able to provide hydrogen gas at a pressure up to 2000 PSI (13.5 MPa). Because of various issues related to hydrogen and oxygen being a combustible mixture, there are limits as to the amount of pressure that an electrolyzer can ultimately generate, which is far less than 10,000 PSI (70 MPa), which is the settled pressure of the fuel cell tank at 15°. These issues include hydrogen purity when high-pressure oxygen is contained in the system and hydrogen cross-over problems through membranes used to separate the gases.
Most of the energy expended when compressing a gas is on a low end of the pressure ramp. This is because the energy verses pressure relationship is a log2 relationship. For example, the theoretical energy needed to compress hydrogen from atmosphere to 70 MPa at 15° C. is 2.01 kWh/kg, and the theoretical energy needed to compress hydrogen to 13.5 MPa at 15° C. is 1.48 kWh/kg. At 13.5 MPa, 74% of the 70 MPa compression work is already done and only 0.53 kWh/kg of the theoretical work is left to be performed. The energy requirement to electrolyze hydrogen is typically 50-60 kWh/kg. 40 kWh/kg is the theoretical limit, so significant improvement in this value is unlikely. Because state-of-the-art electrolyzers can provide hydrogen at about 13.5 MPa, most of the work necessary to get the hydrogen from atmospheric pressure to 70 MPa has already been done at the outlet of the electrolyzer.